>>It is only prudent to shore up your defenses when a country in your region starts a war
I'd actually say it is much more prudent to shore up your defenses BEFORE you need it, not after. Playing catch up is hard to do when the tanks start rolling in.
I find your rationale too simple. It is reasonable to assume that it would have been at the same time unacceptable for the population and perceived as a threat by other nations, should that have been done proactively.
What exactly is 'too simple' about planning for the future? Do you also start shopping for homeowners insurance once the house catches on fire?
Last few decades have been 'relatively' peaceful for many parts of Europe, but given the almost 2 thousand year history of on and off warfare in Europe, nobody should have been surprised for the need for a country to defend itself or its neighbors.
What is 'simple' is assuming that because it is peaceful today, it will always be peaceful.
Since you're comparing a house to a country, resp. a fire to war, and resp. an insurance to diplomatic relationships, then I stand strongly by my initial comment.
Your comment is overly simplistic as it ignores a ton of the signals that nations and leaders emit by doing one thing or the other.
Think of the nuclear treaties: they're here to limit the spread of nuclear weapons, actually adding to the global security.
Si, yes, not beefing up your military can be beneficial to everyone.
> shore up your defenses BEFORE you need it, not after
This needs to be balanced against the public opinion, which is not (AFAIK) very favourable to war-related business in Germany. It is also complicated by the fact that nobody knows when they will be needed, so war spending can be practically increased as a matter of principle, not because war is expected in X years; this makes it more difficult to accept.
This is like the old saying that the best time to plant a tree was 20 years ago. The second best time is today.
No one can change the past, so shoring up defenses now is better than waiting until later, especially when the need might be more acute and it is actually too late.
One can reason that engaging in an arms race is not a good idea neither. It may, on a systemic level accelerate things, and you may end up creating the world you wanted to prevent.
Moreover, everything seems obvious afterwards. But what if Putin had not started rampaging? The opportunity cost of spending 100B on defense if it wasn't needed, is huge. Integrated over decades, that sort of money is (part of) the difference between having a German economy and a Russian economy.
Also, investments age. It may make sense to make upgrades only when you need them.
Those are all maybes. I don't have the answer. But I think it'd good to have in mind that the world is more complex than that.
I'd actually say it is much more prudent to shore up your defenses BEFORE you need it, not after. Playing catch up is hard to do when the tanks start rolling in.